Power steering apparatuses are conventionally utilized which assist operation of a steering wheel by supplying a working oil from an oil pump to a power cylinder coupled to a steering mechanism. The oil pump is driven by an electric motor, and a steering assist force is generated by the power cylinder in accordance with the speed of the electric motor. Since the steering assist force is not required when the steering is not turned, a so-called "stop-and-go control" is conducted, wherein the electric motor is off when the steering assumes a straight travel steering state virtually at a steering angle midpoint and, in response to detection of a steering angle change greater than a predetermined level, the electric motor is actuated.
During a several-millisecond period immediately after the actuation of the electric motor, the target speed of the electric motor is set at an assist starting speed that is greater than an assist speed which is a target speed corresponding to a required steering assist force. This suppresses a delay between the actuation of the electric motor and the generation of the required steering assist force.
As a more urgent steering operation is required, for example, for avoiding a forward obstacle, the delay in application of the steering assist force should be reduced. To this end, Japanese Patent Application No. 8-129060 (1996) (U.S. patent application Ser. No. 08/733,644) previously filed by the applicant proposes that the assist starting speed be determined in accordance with the steering speed.
Since it is impossible to accurately detect a steering urgency level and driver's intention on the basis of the steering speed alone, the arrangement according to the previous patent application cannot always properly determine the assist starting speed. When the steering wheel is suddenly turned at a lower steering urgency level immediately after the start of the steering operation, or when the steering wheel is operated relatively slowly at a higher steering urgency level immediately after the start of the steering operation, for example, the detection of the steering urgency level on the basis of the steering speed is liable to be erroneous.
Although the assist starting speed may be determined in accordance with the time-related change rate of a steering torque, the same problem as encountered by the steering-speed based control is unavoidable.